JP2010116821A - Emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an emission control device that can suppress an amount of nitrogen oxide discharged to the outside without being purified. <P>SOLUTION: The emission control device is configured to execute an adsorption processing for controlling urea adding valves 31 and 41 and a switching valve 20 to add urea only to a second stage catalyst 12 when the condition of the second stage catalyst 12 does not meet a predetermined condition, and flowing exhaust gas G in a first branch passage 5 to adsorb nitrogen oxide in a first stage catalyst 10, and a first stage catalyst reduction processing for adding a required amount of urea for reducing the nitrogen oxide adsorbed in the first stage catalyst 10 by the adsorption processing to the first stage catalyst 10, and flowing the exhaust gas G in the first branch passage 5 for a period of time required for the reduction. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

  In general, a NOx catalyst for purifying NOx (nitrogen oxide) contained in exhaust gas is known as an exhaust purification device disposed in an exhaust system of an internal combustion engine such as a diesel engine. Various types of NOx catalysts are known, and among them, a selective reduction type NOx catalyst that continuously reduces and removes NOx by adding a reducing agent is known. Urea is known as a reducing agent. Usually, a urea aqueous solution mixed with water at a predetermined ratio is injected and supplied into exhaust gas upstream of the catalyst, and urea contained in the urea aqueous solution is hydrolyzed by heat of exhaust gas or the like to generate ammonia. Then, NOx in the exhaust gas is purified by a reducing action of this ammonia by removing oxygen from NOx and returning it to nitrogen on the NOx catalyst.

  Patent Document 1 discloses a technique for reliably purifying NOx contained in exhaust gas of an internal combustion engine from a low temperature region to a high temperature region. Specifically, a NOx adsorption tower is provided on the upstream side of the NOx selective reduction catalyst using ammonia as a catalyst. When the temperature of the exhaust gas is low, the exhaust gas flows into the NOx adsorption tower and the NOx adsorption tower is filled with NOx. To adsorb. On the other hand, when the temperature of the exhaust gas is high, a reducing agent is added to the exhaust gas, and NOx is reduced on the NOx selective reduction catalyst by the added reducing agent.

Japanese Patent Laid-Open No. 07-136464

  By the way, when purifying NOx using the reducing action of ammonia, it is usually necessary to hydrolyze urea to produce ammonia, but it takes some time to hydrolyze urea to produce ammonia. .

  For this reason, if a large amount of NOx flows into the NOx selective reduction catalyst before the urea is hydrolyzed to produce a sufficient amount of ammonia, even if the amount of urea added is increased accordingly, a sufficient amount of ammonia will not be produced. Since a certain amount of time is required until it is generated, NOx may be discharged outside without being purified.

  The present invention has been made in view of the above circumstances, and its object is to perform selective reduction type exhaust purification in which nitrogen oxides contained in exhaust gas discharged from an internal combustion engine are reduced using urea. An object of the present invention is to provide an exhaust emission control device capable of suppressing the amount of nitrogen oxides discharged outside without being purified.

  An exhaust gas purification apparatus for an internal combustion engine according to the present invention is provided in the first and second branch passages that branch in the middle of the exhaust passage of the internal combustion engine and merge again, and by hydrolysis of urea. A pre-stage catalyst for reducing nitrogen oxides contained in the exhaust gas by the reducing action of the generated ammonia; and the exhaust passage downstream of the first and second branch passages; and by hydrolysis of urea A rear catalyst for reducing nitrogen oxides contained in the exhaust gas by a reducing action of the generated ammonia, urea addition means capable of adding urea to the front catalyst and the rear catalyst, and the first and second branches; An adjustment valve capable of adjusting the amount of exhaust gas flowing into the passage, the urea addition means and the adjustment valve are controlled so that urea is supplied only to the rear catalyst when the state of the rear catalyst does not satisfy a predetermined condition. Necessary for reducing the nitrogen oxides adsorbed on the preceding catalyst by the adsorption treatment, and an adsorption treatment for causing the exhaust gas to flow through the first branch passage and adsorbing the nitrogen oxide on the preceding catalyst. Control means for adding a sufficient amount of urea to the first stage catalyst and performing at least a first stage catalyst reduction process for causing the exhaust gas to flow through the first branch path for a time required for the reduction. An exhaust purification device for an internal combustion engine.

  In the above configuration, when the state of the rear catalyst does not satisfy a predetermined condition, the temperature of the rear catalyst is lower than a predetermined temperature, the temperature of the exhaust gas is lower than the predetermined temperature, and the internal combustion engine A configuration including any of cases immediately after start-up can be employed.

  In the above configuration, the control means may employ a configuration in which the exhaust gas is circulated through the first branch path for a time necessary for the reduction of nitrogen oxides adsorbed on the pre-stage catalyst in the pre-stage catalyst reduction process. .

  The said structure can employ | adopt the structure containing the switching valve which distribute | circulates the whole quantity of exhaust gas to one of the said 1st and 2nd branch passages.

  In the above configuration, the urea addition means includes a first urea addition valve provided in an exhaust passage upstream of the preceding catalyst and a second urea addition valve provided in an exhaust passage upstream of the rear catalyst. A configuration including the above can be adopted.

  The said structure WHEREIN: The said urea addition means can employ | adopt the structure provided in the upstream of the said adjustment valve, and including the urea addition valve used in common with the said front | former stage catalyst and the said back | latter stage catalyst.

  ADVANTAGE OF THE INVENTION According to this invention, the selective reduction type exhaust gas purification apparatus which reduces the nitrogen oxide which can suppress the quantity of the nitrogen oxide discharged | emitted outside without purifying using urea is obtained.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, exemplary embodiments of the invention will be described with reference to the accompanying drawings.

  FIG. 1 is a configuration diagram of an exhaust gas purification apparatus for an internal combustion engine according to an embodiment of the present invention.

  The exhaust purification device 1 is used for purifying nitrogen oxides (NOx) contained in exhaust gas generated by an internal combustion engine such as a diesel engine, for example.

  As shown in FIG. 1, the exhaust purification device 1 includes a first branch passage 5 and a second branch passage 7 that branch in the middle of the exhaust passage 3A on the upstream side of the internal combustion engine and merge again, and a first branch passage. 5, a post-stage catalyst 12 provided in the exhaust passage 3 </ b> B downstream of the first branch passage 5 and the second branch passage 7, and a pre-stage urea capable of adding urea to the pre-stage catalyst 10. An addition valve 31, a rear urea addition valve 41 capable of adding urea to the rear catalyst 12, a switching valve 20 connected to the exhaust passage 3A, the first branch passage 5, and the second branch passage 7, and control means As an electronic control unit (hereinafter referred to as ECU) 100, exhaust temperature sensors 61 and 62 provided on the upstream side of the front catalyst 10 and the rear catalyst 12, respectively, and on the downstream side of the front catalyst 10 and the rear catalyst 12, respectively. NOx sensor 51 provided, And a 2.

  The pre-stage catalyst 10 and the post-stage catalyst 12 are known selective reduction catalysts (SCR: Selective Catalytic Reduction) that use a catalyst species such as zeolite or vanadium to reduce nitrogen oxides by a so-called urea selective reduction method. That is, on the catalyst, nitrogen oxides contained in the exhaust gas are reduced and purified by the reducing action of ammonia generated by hydrolysis of urea. These front-stage catalyst 10 and rear-stage catalyst 12 are heated to a predetermined minimum active temperature (for example, 200 ° C.) or higher, and the supplied urea is hydrolyzed to produce a sufficient amount of ammonia. The oxide can be reduced. Further, since the base catalyst 10 and the post catalyst 12 have a void structure in which the base material has a large number of pores, they have an adsorbing effect on the substances flowing into them. In particular, when zeolite or the like is used as the catalyst species, the adsorption effect on nitrogen oxides and ammonia is high.

  The upstream urea addition valve 31 is connected to a supply device 32 for supplying a urea aqueous solution, and the supply device 32 is connected to a tank 33 for storing a urea aqueous solution in which urea is mixed with water at a predetermined ratio. The pre-stage urea addition valve 31 can inject urea from the upstream of the pre-stage catalyst 10 in the form of a urea aqueous solution. The pre-stage urea addition valve 31 and the supply device 32 are controlled by the ECU 100.

  The post-stage urea addition valve 41 is connected to a supply device 42 for supplying a urea aqueous solution, and the supply device 42 is connected to a tank 43 for storing a urea aqueous solution in which urea is mixed with water at a predetermined ratio. The post-stage urea addition valve 41 can inject urea from the upstream of the post-stage catalyst 12 in the form of an aqueous urea solution. The post-stage urea addition valve 41 and the supply device 42 are controlled by the ECU 100.

  The switching valve 20 is composed of, for example, an electromagnetic valve, and the entire amount of the exhaust gas G flowing through the exhaust passage 3A is supplied to one of the first branch passage 5 and the second branch passage 7 in accordance with a control command from the ECU 100. It is provided for distribution. Instead of the switching valve 20, it is possible to use an adjustment valve that can continuously adjust the amount of exhaust gas G flowing into the first branch passage 5 and the second branch passage 7. In this case, the ratio of the exhaust gas G flowing through the first branch passage 5 and the second branch passage 7 can be arbitrarily changed by a control command from the ECU 100.

  The ECU 100 includes hardware such as a CPU, ROM, RAM, input / output port, and storage device, and necessary software. The ECU 100 receives detection signals from the exhaust temperature sensors 61 and 62 and the NOx sensors 51 and 52. The front-stage urea addition valve 31 and the supply device 32, the rear-stage urea addition valve 41, the supply device 42, and the switching valve 20 are controlled. The specific processing content by the ECU 100 will be described later.

  The NOx sensors 51 and 52 generate an electrical signal corresponding to the amount of nitrogen oxide contained in the exhaust gas G and output it to the ECU 100.

  The exhaust temperature sensors 61 and 62 generate an electrical signal corresponding to the temperature of the exhaust gas G and output it to the ECU 100.

  Next, an example of control by the ECU 100 will be described with reference to a flowchart shown in FIG.

  When the internal combustion engine is started, the ECU 100 starts adding urea to the rear catalyst 12 so that the rear catalyst 12 can purify nitrogen oxides (NOx), and the rear catalyst 12 continuously adds urea. Is added, but urea is not added to the pre-stage catalyst 10. Further, immediately after the start, the switching valve 20 is in a state in which the exhaust gas G flows into the first branch passage 5 (pre-stage catalyst 10) side. In this state, the processing routine shown in FIG. 2 is executed every predetermined time.

  First, it is determined whether NOx can be reduced by the rear catalyst 12 (step S1). That is, in order to reduce NOx, it is determined whether the state of the post-catalyst 12 satisfies a predetermined condition. The state in which the rear catalyst 12 is not reducible is, for example, when the rear catalyst 12 is not activated immediately after the start of the internal combustion engine, or when the urea supplied to the rear catalyst 12 is not yet sufficiently hydrolyzed. This is a case where the amount of ammonia necessary for the reduction is not generated. Therefore, for example, it is determined whether the temperature of the exhaust gas G is equal to or higher than a predetermined temperature, whether the temperature of the post-catalyst 12 is higher than a predetermined temperature, whether the internal combustion engine is started and a predetermined period has elapsed. Then, it is determined whether the rear catalyst 12 can be reduced. Note that the method for determining whether the rear catalyst 12 can be reduced is not limited to these examples.

  If the rear catalyst 12 is not yet in a reducible state, it is determined whether the front catalyst 10 can adsorb NOx (step S2). For example, it is determined whether the NOx adsorption amount that the pre-stage catalyst 10 has adsorbed so far exceeds a predetermined amount that is within the range in which the pre-stage catalyst 10 can be adsorbed. It can be determined that the catalyst 10 can adsorb NOx.

  The NOx adsorption amount that the pre-stage catalyst 10 has adsorbed to date is, for example, provided downstream of the pre-stage catalyst 10 and the amount of NOx contained in the exhaust gas G discharged from the internal combustion engine calculated from the operating conditions of the internal combustion engine. It can be calculated based on the amount of NOx detected by the obtained NOx sensor 51.

  In step S2, when it is determined that the front stage catalyst 10 can be adsorbed, the switching valve 20 is switched so that the exhaust gas G flows into the first branch passage 5 (front stage catalyst 10). When the switching valve 20 is already set on the first branch passage 5 side, the state is maintained. As a result, the exhaust gas G flows into the front catalyst 10, and NOx contained in the exhaust gas G is adsorbed by the front catalyst 10. Further, while the front catalyst 10 is adsorbing NOx, the rear catalyst 12 is heated and urea is continuously added, and the urea is hydrolyzed to generate ammonia, so that NOx can be reduced. Migrate to

  Next, the NOx adsorption amount that the pre-catalyst 10 has adsorbed so far is calculated by the method as described above (step S4), and the process is terminated.

  If it is determined in step S2 that the pre-stage catalyst 10 is not in an adsorbable state, that is, if the pre-stage catalyst 10 has already adsorbed a predetermined amount of NOx and is determined to be saturated or nearly saturated, The amount of NOx adsorbed by the catalyst 10 so far is calculated (step S7).

  Then, the amount of urea added to reduce all the NOx adsorbed by the former catalyst 10 so far is calculated (step S6). Further, a reduction time necessary for reducing all of the NOx adsorbed by the pre-stage catalyst 10 so far is calculated.

  After calculating the required urea addition amount and the required reduction time, the switching valve 20 is switched so that the entire amount of the exhaust gas G flows through the second branch passage 7 (step S8). As a result, the exhaust gas G is directly supplied to the rear catalyst 12 through the second branch passage 7 without passing through the front catalyst 10, and the NOx contained in the exhaust gas G is purified by the rear catalyst 12. Since the post-stage catalyst 12 has already shifted to a reducible state while the pre-stage catalyst 10 is adsorbing NOx, NOx can be prevented from passing through the post-stage catalyst 12 without being reduced.

  If it is determined in step S1 that the rear catalyst 12 is in a reducible state, it is determined whether NOx adsorbed on the front catalyst 10 remains (step S9). For example, it is determined from zero whether the NOx adsorption amount calculated in step S4 or step S5 exists.

  When it is determined that NOx remains in the pre-stage catalyst 10, the amount of urea calculated in step S6 is injected from the pre-stage urea addition valve 31 (step S11). Thereby, in the pre-stage catalyst 10, urea is hydrolyzed to generate ammonia. Further, it is determined whether the reduction time calculated in step S7 has elapsed (step S12). Note that the timing of urea injection from the upstream urea addition valve 31 can be variously changed, and a certain amount of urea may be injected from the upstream urea addition valve 31 during the reduction time calculated in step S7. It is also possible to inject a necessary amount of urea before a sufficient reduction time elapses.

  When a necessary amount of urea is supplied to the pre-stage catalyst 10 and the necessary reduction time has elapsed, NOx adsorbed on the pre-stage catalyst 10 is almost completely reduced by ammonia, and the ammonia produced in the pre-stage catalyst 10 is Almost all is used for the reduction of NOx, and ammonia is hardly adsorbed on the pre-stage catalyst 10.

  At this time, the NOx adsorption amount of the front catalyst 10 is cleared (zero) (step S13), and the switching valve 20 is switched to the second branch passage 7 side (step S14). Thereby, since the ammonia and NOx are hardly adsorbed, the pre-stage catalyst 10 can be prepared for the next NOx adsorption with the NOx adsorption effect (adsorption capacity) maximized. Further, by making the amount of urea added to the pre-stage catalyst 10 an amount necessary and sufficient for reduction, the amount of ammonia adsorbed on the pre-stage catalyst 10 can be suppressed, and the adsorption effect of the pre-stage catalyst 10 at the time of NOx adsorption is maximized. It becomes possible to raise it to the limit.

  FIG. 3 is a diagram showing a configuration of an exhaust emission control device according to another embodiment of the present invention. In FIG. 3, the same reference numerals are used for the same components as those in the exhaust gas purification apparatus shown in FIG.

  The exhaust purification apparatus 1A shown in FIG. 3 includes a urea addition valve 131, a supply device 132, and a tank 133 that are commonly used to add urea to the front catalyst 10 and the rear catalyst 12. Thereby, a structure can be simplified and apparatus cost can be reduced.

  Although the case where the switching valve 20 is used as the regulating valve of the present invention has been described in the above embodiment, the present invention is not limited to this. When an adjustment valve that can continuously change the ratio of the exhaust gas G flowing into the first branch passage 5 and the second branch passage 7 instead of the switching valve 20 is used, for example, step S3, In S8, S10, S14, etc., the exhaust gas G is not allowed to flow into one of the first branch passage 5 and the second branch passage 7, but can be allowed to flow through both passages. At this time, the ratio of the exhaust gas G flowing through the first branch passage 5 and the second branch passage 7 can be appropriately adjusted.

It is a figure showing composition of an exhaust-air-purification device concerning one embodiment of the present invention. It is a flowchart which shows an example of the process sequence by ECU of FIG. It is a figure which shows the structure of the exhaust gas purification apparatus which concerns on other embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Exhaust purification apparatus 3A, 3B ... Exhaust passage 5 ... 1st branch passage 7 ... 2nd branch passage 10 ... Pre-stage catalyst 12 ... Post-stage catalyst 20 ... Switching valve (regulation valve)
31 ... Pre-stage urea addition valve (urea addition means)
32 ... Supply device 33 ... Tank 41 ... Post-stage urea addition valve (urea addition means)
42 ... Supply device 43 ... Tanks 51, 52 ... NOx sensors 61, 62 ... Exhaust temperature sensor 100 ... ECU (control means)

Claims (6)

First and second branch passages which branch in the middle of the exhaust passage of the internal combustion engine and merge again;
A pre-stage catalyst that is provided in the first branch passage and reduces nitrogen oxides contained in the exhaust gas by a reducing action of ammonia generated by hydrolysis of urea;
A downstream catalyst that is provided in the exhaust passage downstream of the first and second branch passages and that reduces nitrogen oxides contained in the exhaust gas by the reducing action of ammonia generated by hydrolysis of urea;
Urea addition means capable of adding urea to each of the front catalyst and the rear catalyst;
An adjustment valve capable of adjusting the amount of exhaust gas flowing into the first and second branch passages;
By controlling the urea addition means and the regulating valve, when the state of the rear catalyst does not satisfy a predetermined condition, urea is added only to the rear catalyst and the exhaust gas is circulated through the first branch passage. An adsorption process for adsorbing nitrogen oxides on the pre-stage catalyst, and an amount of urea necessary for reducing the nitrogen oxides adsorbed on the pre-stage catalyst by the adsorption process is added to the pre-stage catalyst and the exhaust gas is added to the first stage catalyst. Control means for executing at least a pre-catalyst reduction process for flowing through one branch path;
An exhaust emission control device for an internal combustion engine, comprising:   When the state of the rear catalyst does not satisfy a predetermined condition, the temperature of the rear catalyst is lower than a predetermined temperature, the temperature of the exhaust gas is lower than the predetermined temperature, and the internal combustion engine is immediately after starting The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein any one of the cases is included.   2. The control means causes the exhaust gas to flow through the first branch path for a time required for the reduction of nitrogen oxides adsorbed on the front catalyst in the front catalyst reduction treatment. Or an exhaust gas purification apparatus for an internal combustion engine according to 2 or 2,   The said adjustment valve includes the switching valve which distribute | circulates the whole quantity of the exhaust gas which distribute | circulates the said exhaust passage to one of the said 1st and 2nd branch passage, The any one of Claim 1 thru | or 3 characterized by the above-mentioned. Exhaust gas purification device for internal combustion engine.   The urea addition means includes a first urea addition valve provided in an exhaust passage on the upstream side of the upstream catalyst and a second urea addition valve provided in an exhaust passage on the upstream side of the rear catalyst. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 4, wherein   5. The urea addition unit according to claim 1, wherein the urea addition unit includes a urea addition valve that is provided upstream of the regulating valve and is commonly used for the front-stage catalyst and the rear-stage catalyst. An exhaust gas purification apparatus for an internal combustion engine as described.

Images